Solid oxide fuel cells (“SOFCs” below) are fuel cells which operate at a relatively high temperature in which, using an oxide ion conducting solid electrolyte as electrolyte, with electrodes attached to both sides thereof, fuel gas is supplied to one side thereof and oxidizer (air, oxygen, or the like) is supplied to the other side thereof.
Japanese Published Unexamined Patent Application 2012-3850 (Patent Document 1) discloses a solid oxide fuel cell. In this solid oxide fuel cell, when a fuel cell operating at a high temperature is turned off, air is supplied to the air electrode side of the fuel cell stack while continuing to supply a small amount of fuel and fuel-reforming water, and the temperature inside the fuel cell module is reduced by the cooling effect of this air. I.e., in this fuel cell, during the stopping step fuel continues to be supplied even after the extraction of power from the fuel cell module is stopped, while at the same time the fuel cell stack is cooled by delivering a large volume of cooling air. Next, when the cell stack temperature has been reduced to less than the fuel cell oxidation temperature, the supply of fuel is stopped, after which only the supply of cooling air is continued until the temperature drops sufficiently, and the fuel cell is safely turned off.
A fuel cell which performs a “shutdown stop” is also known, whereby in the stopping step, power is extracted and the supply of fuel, fuel reforming water, and generating air (air fed to the air electrode side) is completely stopped.
Japanese Published Unexamined Patent Application 2010-27579 (Patent Document 2) discloses a fuel cell system. In this fuel cell system, during an emergency stop the feed pump for supplying fuel to the reformer, the reform water pump for supplying water for steam reforming, and the air blower for feeding air to the air electrode side of the cell stack are stopped. Thereafter, when the feed pump and the reforming water pump are restarted under emergency stop operation control, fuel gas which had been adsorbed by the adsorber is fed to the reformer and steam reforming is carried out using water supplied from the reform water pump, even if the supply of fuel gas from the fuel supply source is cut off. By this means, reforming fuel is supplied to the cell stack electrode over a predetermined period even after the supply of fuel gas is cut off, and oxidation of fuel electrodes by reverse flow of air is prevented.
Furthermore, Japanese Published Unexamined Patent Application 2012-138186 (Patent Document 3) discloses a high temperature-triggered fuel cell system. In this high temperature-triggered fuel cell system, during an emergency stop the raw fuel pump for supplying fuel gas is stopped and the reforming pump for supplying water to the reformer is activated. When the reforming water pump is activated, the water supplied expands in volume due to vaporization inside the reformer. Therefore even if the supply of raw fuel gas from the fuel supply source is cut off, the fuel gas remaining in the fuel gas supply line downstream of the reformer is pushed toward the fuel cell (cell stack) side by the pressure of the volumetrically expanded steam. Oxidation of the fuel electrode by a reverse flow of air is thus prevented.